Radio frequency (RF) communication, such as that used in mobile phones, requires RF filters that each passes a desired frequency and block all other frequencies. The core of the RF filter is an acoustic resonator.
Ever greater data traffic results in a drive to higher frequencies and more filters per mobile phone. To keep such phones from becoming larger, RF filters are required to be ever smaller. To prevent battery drainage and generation of heat requiring dissipation, filters having low power consumption are required.
Each RF filters includes an array of acoustic resonators. The quality of each resonator is given by its Q factor, which is a dimensionless parameter that describes how under-damped an oscillator or resonator is, and characterizes a resonator's bandwidth relative to its center frequency, which is the ratio of the energy stored to the power dissipated. The next generation of mobile phones requires quality resonators and filters having high Q factors indicating low energy loss during operation. This translates to a lower insertion loss and a steeper skirt for “sharper” differentiation to nearby bands.
One type of resonator is the Bulk-acoustic-wave (BAW) resonator. The electrical impedance of a BAW resonator has two characteristic frequencies: the resonance frequency fR and anti-resonance frequency fA. At fR, the electrical impedance is very small, whereas at fA the electrical impedance is very large. Filters are made by combining several resonators. A typical arrangement includes a “half-ladder” architecture comprising resonators in series and shunt. The shunt resonator is shifted in frequency with respect to the series resonator. When the resonance frequency of the series resonator equals the anti-resonance frequency of the shunt resonator, the maximum signal is transmitted from the input to the output of the device. At the anti-resonance frequency of the series resonator, the impedance between the input and output terminals is high and the filter transmission is blocked. At the resonance frequency of the shunt resonator, any current flowing into the filter section is shorted to ground by the low impedance of the shunt resonator so that the BAW filter also blocks signal transmission at this frequency. The frequency spacing between fR and fA determines the filter bandwidth.
For frequencies other than the resonance and anti-resonance frequencies, the BAW resonator behaves like a Metal-Insulator-Metal (MIM) capacitor. Consequently, far below and far above these resonances, the magnitude of the electrical impedance is proportional to 1/f where f is the frequency. The frequency separation between fR and fA is a measure of the strength of the piezoelectric effect in the resonator that is known as the effective coupling coefficient—represented by K2eff. Another way to describe the effective coupling coefficient is as a measure of the efficiency of the conversion between electrical and mechanical energy by the resonator (or filter). It will be noted that the electromechanical coupling coefficient is mainly a material's related property that defines the K2eff for the piezoelectric film.
The level of performance of a filter is defined by its factor of merit (FOM) which is defined as FOM=Q*K2eff.
For practical applications, high K2eff and Q factor values are both desirable. However, there is a trade-off between these parameters. Although K2eff is not a function of frequency, the Q-value is frequency dependent and therefore the FOM (Factor of Merit) is also a function of frequency. Hence the FOM is more commonly used in filter design than in the resonator design.
In many applications, a lowering in the K2eff of a device can be tolerated to achieve a high Q factor since a small sacrifice in K2eff may give a large boost to the Q value. However, the opposite approach of sacrificing Q-value to obtain a design having an adequate K2eff is not feasible.
The K2eff of a resonator can be enhanced by choosing a high acoustic impedance electrode, and it can also be enhanced by compromising other parameters such as increasing the thicknesses of the electrode and passivation layer.
The highest performance (i.e. highest FOM) type of bulk acoustic wave resonator or filter is the free-standing bulk acoustic resonator or FBAR. In the traditional FBAR resonator a free-standing bulk acoustic membrane which is supported only around its edge is used. An air cavity is provided between the bottom electrode and the carrier wafer. There is a need for improved FBAR resonators and the present invention addresses this need.